<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <title>虚线setLineDash  </title>
</head>
<body>
<canvas id="canvas" width="200" height="200"></canvas>
<script>
  var ctx = document.getElementById('canvas').getContext('2d');

  ctx.beginPath();
  ctx.arc(50, 50, 30, 0, Math.PI*2, true);

  ctx.arc(50, 50, 5, 0, Math.PI*2, true);
  ctx.fill("evenodd");

</script>
</body>
</html>